The engineer in me tells me that when they get complacent and stop doing safety checks, get reckless or one of those bands lets go, your going to drill your body into the upright that is still attached at Warp Speed!!!! theres a reason amusement rides, and bungee operators need to be licensed, and registered.

I see a lot going wrong with a home made set up.

later all (and I am no stranger to danger, and have always been the first to try stuff out)

I don't know these guys, and I wouldn't trust them with my safety without looking and knowing what they did. But I also would trust my homemade stuff more than licensed bungee/roller coaster/whatever guys too. Just because homemade stuff is usually crap doesn't mean it always is. And what they did wouldn't take much more than some basic physics.